1. Field of the Invention
The present invention relates to a method of cleaning a wafer. More particularly, the present invention relates to a method of cleaning a wafer after an etching process.
2. Description of the Related Art
Damascene process is a meticulous technique for embedding metallic interconnection inside an insulation layer. The method includes etching a dielectric layer on a substrate to form a plurality of trenches and via openings according to the required metallic line pattern and the locations of vias. Then, a metallic layer is deposited on the substrate to fill up the trenches and the openings so that the metallic lines and the vias are simultaneously formed. Because using the dual damascene process can prevent overlay errors and process deviation problems that result from the typical process of forming a via before forming a metallic conducting wire in a photolithographic process, the device has a higher reliability and the processing capability is increased. Therefore, with the demand for a higher level of integration, dual damascene process has gradually become a widely adopted fabricating technique in semiconductor industry.
However, a mask layer fabricated from metallic material such as titanium nitride is often used as an etching mask in the process of etching the dielectric layer to form trenches and via openings. The etching gas includes CxFy compounds, and the CxFy compounds are bonding to form the long carbon chain polymer during the etching process. The long carbon chain polymer or the polymer formed between the long carbon chain polymer and the metallic ions freed from plasma bombardment and other reactive gases or photoresist material will be deposited on the wafer surface. In addition, after the etching process, the residual fluorine will attach to the wafer surface and react with the metallic ions within the metallic mask layer to form metallic fluorides such as titanium fluoride. These residual materials such as the polymers and the metallic fluorides may produce some adverse effects on the electrical properties of the device leading to a drop in device performance. Thus, there is a need to remove the polymers and metallic fluorides after the etching process.
A conventional technique for removing these polymers is a three-stage cleaning process that includes performing a wet cleaning, a dry cleaning and a wet cleaning processes in sequence. The first wet cleaning process is carried out to soften the polymers and detach the polymers from the wafer surface. The dry etching process is carried out to miniaturize the polymers. Finally, the second wet cleaning process is carried out to remove the remaining polymers.
Yet, the cleaning solution in the wet cleaning processes is mostly an alkaline solution that will cause some damage to the metallic layer in the substrate exposed by the openings.
Moreover, the performance of the aforementioned method of removing the polymers is poor at removing metallic fluorides from the wafer. Therefore, some residual metallic fluoride will still remain on the wafer after performing the polymer removing process and consequently produce some adverse effects on the device.
In addition, the three-stage cleaning process for removing polymers on the wafer is rather complicated. Ultimately, this will affect productivity.